View full sizeAFP PHOTO / ALEXANDER KLEINALEXANDER KLEIN/AFP/GettyImagesAfghan youth walk past an old tank at sunset in Kabul on Aug. 9. The war-torn country, beset by a decade-long Taliban insurgency and rampant corruption, is one of the world's poorest, with more than a third of the population living below the poverty line in 2008.

KABUL, Afghanistan — An Afghan police officer shot and killed three U.S. Marines after sharing a meal with them before dawn Friday and then fled into the desolate darkness of southern Afghanistan, the third attack on coalition forces by their Afghan counterparts in a week.

Thirty-one coalition service members have now died this year at the hands of Afghan forces or insurgents disguised in Afghan uniforms, according to NATO — a dramatic rise from previous years.

The assaults have cast a shadow of fear and mistrust over U.S. efforts to train Afghan soldiers and police more than 10 years after the U.S.-led invasion to topple the Taliban’s hard-line Islamist regime for sheltering al Qaeda’s leadership. The attacks also raise further doubts about the quality of the Afghan forces taking over in many areas before most international troops leave the country in 2014.

Friday’s deadly shooting took place in the volatile Sangin district of Helmand province, said U.S. military spokeswoman Maj. Lori Hodge. Sangin was a Taliban stronghold for years and has one of the highest concentrations of improvised explosive devices in the country.

TALIBAN CLAIMS ROLE

A U.S. Defense Department official confirmed that the dead Americans were Marine Special Operations Forces. He spoke on the condition of anonymity because the family notification process was not complete.

Sangin’s district chief and the Taliban identified the gunman as Asadullah, a member of the Afghan National Police who was helping the Marines train the Afghan Local Police.

Taliban spokesman Qari Yousef Ahmadi said by telephone that the attacker joined the insurgency after the shooting.

"Now, he is with us," Ahmadi said.

The district chief, Mohammad Sharif, said the shooting happened at a police checkpoint after a joint meal and a security meeting. The meal took place before dawn because of Ramadan, the month when Muslims abstain from food during daylight hours.

Compared with the 25 attacks this year that have killed 31 foreign troops, there were 11 such attacks and 20 deaths last year, according to an Associated Press count. Each of the previous two years saw five such attacks.

OTHER ATTACKS

The NATO coalition says it takes the rise in "green-on-blue" attacks seriously but insists they are not a sign of trouble for the plan to hand over security to Afghan forces.

"We are confident that those isolated incidents will have no effect on transition or on the quality of our forces," said Brig. Gen. Gunter Katz, a spokesman for NATO troops.

White House spokesman Jay Carney said President Obama remains committed to his timeline to transfer the security lead to Afghan forces by the end of 2013.

"It is too early to say this latest incident is part of a stepped-up effort by insurgents," Carney said Friday. But he added that the administration considers the attacks serious and that U.S. commanders are evaluating Taliban tactics.

On Tuesday, two gunmen wearing Afghan army uniforms killed a U.S. soldier and wounded two others in Paktia province in the east. And on Thursday, two Afghan soldiers tried to gun down a group of NATO troops outside a military base in eastern Afghanistan. No international forces died, but one of the attackers was killed as NATO forces shot back.

The U.S. government identified four Americans who were killed along with an Afghan civilian in a twin suicide attack in eastern Kunar province Wednesday: USAID foreign service officer Ragaei Abdelfattah, Air Force Maj. Walter D. Gray of Conyers, Ga.; Army Maj. Thomas E. Kennedy of West Point, N.Y.; and Command Sgt. Maj. Kevin J. Griffin of Laramie, Wyo.